When handling volatile liquids such as hydrocarbons including gasoline and kerosene, air-volatile liquid vapor mixtures are readily produced. The venting of such air-vapor mixtures directly into the atmosphere results in significant pollution of the environment and a fire or explosion hazard. Accordingly, existing environmental regulations require the control of such emissions.
As a consequence, a number of processes and apparatus have been developed and utilized to recover volatile liquids from air-volatile liquid vapor mixtures. Generally, the recovered volatile liquids are liquified and recombined with the volatile liquid from which they were vaporized thereby making the recovery process more economical.
The initial vapor recovery systems utilized in the United States in the late 1920's and early 1930's incorporated a process combining compression and condensation. Such systems were originally only utilized on gasoline storage tanks. It wasn't until the 1950's that local air pollution regulations began to be adopted forcing the installation of vapor recovery systems at truck loading terminals. Shortly thereafter, the "clean air" legislation activity of the 1960's, which culminated in the Clean Air Act of 1968, further focused nationwide attention on the gasoline vapor recovery problem. As a result a lean oil/absorption system was developed. This system dominated the marketplace for a short time.
Subsequently, in the late 1960's and early 1970's cryogenic refrigeration systems began gaining market acceptance (note, for example, U.S. Pat. No. 3,266,262 to Moragne). While reliable, cryogenic systems suffer from a number of shortcomings including high horsepower requirements. Further, such systems require relatively rigorous and expensive maintenance to function properly. Mechanical refrigeration systems also have practical limits with respect to the amount of cold that may be delivered, accordingly, the efficiency and capacity of such systems is limited. In contrast, liquid nitrogen cooling systems provide more cooling than is required and are prohibitively expensive to operate for this type of application.
As a result of these shortcomings, alternative technology was sought and adsorption/absorption vapor recovery systems were more recently developed. Such a system is disclosed in a number of U.S. Patents including, for example, U.S. Pat. No. 4,276,058 to Dinsmore, the disclosure of which is fully incorporated herein by reference. Such systems utilize a bed of solid adsorbent selected, for example, from silica gel, certain forms of porous mineral such as alumina and magnesia, and most preferably activated charcoal. These adsorbents have an affinity for volatile hydrocarbon liquids. Thus, as the air-hydrocarbon vapor mixture is passed through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed. The resulting residue gas stream comprising substantially hydrocarbon-free air is well within regulated allowable emission levels and is exhausted into the environment.
It should be appreciated that the bed of adsorbent used in these systems is only capable of adsorbing a certain amount of hydrocarbons before reaching capacity and becoming ineffective. Accordingly, the bed must be periodically regenerated to restore the carbon to a level where it will effectively adsorb hydrocarbons again. This regeneration of the adsorbent is a two step process.
The first step requires a reduction in the total pressure by pulling a vacuum on the bed that removes the largest amount of hydrocarbons. The second step is the addition of a purge air stream that passes through the bed. The purge air polishes the bed so as to remove substantially all of the previously adsorbed hydrocarbons. These hydrocarbons are then pumped to an absorber tower wherein lean oil or other nonvolatile liquid solvent is provided in a countercurrent flow relative to the hydrocarbon rich air-hydrocarbon mixture being pumped from the bed. The liquid solvent condenses and removes the vast majority of the hydrocarbons from that mixture and the residue gas stream from the absorber tower is recycled to a second bed of adsorbent while the first bed completes regeneration.
For best efficiency of operation, it should be appreciated that the adsorbent bed must be quickly regenerated to restore the ability of the bed to adsorb volatile liquid vapors. This can best be accomplished by maximizing the performance of the vacuum pump. This may be achieved by operating the vacuum pump at cooler temperatures. Further, the absorber fluid, e.g. lean oil or other nonvolatile liquid solvent, must provide as rapid and complete a recovery of volatile liquid vapor by condensation as possible. This may be achieved by delivering absorber fluid at cooler temperatures to the absorber tower.
Prior art vapor recovery systems do not provide any effective means for optimizing the use of the available "cooling potential" of the absorber fluid or lean oil in the storage tank to achieve these important ends. For example, in the Dinsmore patent absorber fluid or lean oil from the storage tank is directly introduced into both the absorber tower and heat exchanger. The "spent" absorber fluid is, however, then returned from the heat exchanger to the absorber tower. Thus, the heat transferred to the absorber fluid by the heat exchanger is reintroduced into the air-hydrocarbon vapor mixture being processed in the system at the absorber tower. The resulting increase in temperature in the absorber tower impedes efficient condensation and, therefore, liquid vapor recovery. Accordingly, more vapor remains in the air stream exhausted from the absorber tower. This vapor requires subsequent recovery in the second adsorbent bed. Bed capacity is, of course, used in the recovery of this vapor. It should therefore be appreciated that the prior art approach as disclosed in Dinsmore for the circulation of absorber fluid causes a dual negative impact: that is, a reduction in both absorber tower and adsorbent bed vapor recovery efficiency.
It has also been proposed in the prior art to route the absorber fluid from the storage tank direct to the absorber tower. The spent absorber fluid is then directed from the absorber tower through the heat exchanger before returning to the storage tank. As the absorber fluid passes through the absorber tower it contacts vapor delivered from the vacuum pump at temperatures up to 120.degree. F. Hence, the absorber fluid from the absorber tower has absorbed a significant amount of heat and, therefore, cannot provide effective cooling of the vacuum pump. Accordingly, the vacuum pump runs hotter at reduced efficiency. As a result, more time is required to regenerate the bed to the desired level. System productivity is, therefore, impaired and greater energy is used in running the vacuum pump over the longer operating cycle required to complete regeneration.
A need is, therefore, identified for a new and improved approach for enhancing the efficiency of a combined adsorption/absorption vapor recovery system.